Various metal casting processes, such as investment (or "lost wax") casting are well known for the fabrication of metal objects. This process requires several steps, the first of which is to create or provide a pattern or shape to be replicated. The pattern, often made of a heat destructible material such as wax or resin, is used to make a mold that is then used to form cast metal articles.
Typically, several wax patterns are joined together on a "tree" to enable the simultaneous manufacture of several parts. The tree is usually a solid wax or resin tube that has side walls to which a stem of each pattern is joined to form a cluster. The tree defines what will become a gate leading to passages for allowing molten metal to travel through the mold to each cluster and part pattern. Once all of the patterns are joined to the tree, the cluster is coated with one or more coats of a refractory material. This can be accomplished by dipping the pattern/tree assembly in a ceramic slurry.
After the slurry dries, fabrication of a shell or mold is completed by heating the slurry coated pattern/tree to cure or harden the ceramic and to burn-out the solid patterns and the tree. Molten metal is then poured into the shell so that it fills each of the cavities formerly occupied by the patterns and the tree. After the metal has cooled and hardened, the shell is fractured and removed, and the cast metal parts are severed from the metal tree. The cast parts are then subjected to post-machining, grinding off the gates, bead blasting, and polishing, as required.
The above-described burn-out step for removing heat destructible pattern/tree elements from a ceramic shell is commonly performed in an oven or furnace. Although resin is readily destroyed in this manner, it sometimes expands and cracks the shell while being heated. Wax is even more likely to expand and crack the shell. If the pattern/tree includes voids or hollow regions filled with gas, expansion of the gas can also cause the pattern to expand and the shell to crack. Thus, regardless of whether wax or resin are used individually or in combination to provide a solid or void-filled pattern/tree, a burn-out step performed in a furnace can cause substantial loss of ceramic shells due to pattern/tree material expansion or due to trapped gas expansion.
When a pressure differential exists between the interior and the exterior of the ceramic shell as the shell is being heated, regardless of how the pressure differential is created, the pressure differential can cause the shell to crack. To compensate for, or eliminate pressure differentials, it is known to create vents or bleeders in ceramic shells by filing down a portion of the ceramic shell to reveal or to create a pathway for low resistance airflow. Alternatively, a vent can be created in the ceramic shell by clearing slurry from a surface region of the pattern each time the pattern is dipped and before the slurry dries.
Regardless of how the hole or vent is formed, the hole must be repaired or sealed after the burn-out step and prior to metal pouring. The repair process, however, is labor intensive and can cause shell failure. Furthermore, the repair material may become dislodged, causing the hole to partially or completely reopen prior to, during, or after the step of pouring molten metal into the shell. This can result in spilled metal and a defective casting.
It is desirable to provide additional investment casting techniques that address the challenges of reducing mold failure, and more particularly the problems associated with hollow patterns, pattern removal, and pressure differentials.